This disclosure generally relates to high-lift leading edge flaps for the wings of an aircraft, and more particularly to a Krueger leading edge flap having at least a stowed position for high-speed cruise operation and a forwardly extended position in which the forward edge of the Krueger flap forms an aerodynamic slot with the leading edge of the aircraft wing for takeoff and landing.
Modern aircraft often use a variety of high-lift leading and trailing edge devices to improve high-angle of attack performance during various phases of flight, including takeoff and landing. Existing leading edge devices include leading edge slats and Krueger flaps.
Current leading edge slats generally have a stowed position in which the slat forms a portion of the leading edge of the wing, and one or more deployed positions in which the slat extends forward and down to increase the camber and/or planform area of the wing. The stowed position is generally associated with low drag at low angles of attack and can be suitable for cruise and other low angle of attack operations. The extended position(s) is/are generally associated with improved airflow characteristics over the aircraft's wing at higher angles of attack. Typical leading edge slat designs include arrangements in which the leading edge device retracts in an aft direction to form the leading edge of the wing when stowed.
Krueger flaps have generally the same function as leading edge slats, but rather than retracting aft to form the leading edge of the cruise wing, Krueger flaps are hinged, and typically fold into the lower surface of the wing when stowed. When deployed, Krueger flaps extend forward from the under surface of the wing, increasing the wing camber and maximum coefficient of lift.
In the case of a typical Krueger flap, a slot or gap is created between the flap and the wing as the flap is extended forward. During certain operating conditions, air can flow through this slot to energize the airflow over the upper surface of the wing, and improve overall airflow characteristics over the wing.
For some airplane applications, such as extended laminar flow or high cruising speeds, it may be advantageous to design a wing with a small leading edge radius. A small leading edge radius can cause premature flow separation (stall) on the wing surface, even when a Krueger flap is present. This has unfavorable consequences in aerodynamics, since flow separation can significantly decrease lift and increase drag of the wing.
Existing leading edge devices turn the air flow as it moves aft. Turning the aft-moving flow before it reaches the wing leading edge is a necessary condition for delaying stall of a sharp-nosed wing, but it may not be sufficient. This is particularly true if the leading edge device must be positioned high relative to the wing leading edge, as is required for some configurations.